2,639 research outputs found
Lee-Yang zero analysis for the study of QCD phase structure
We comment on the Lee-Yang zero analysis for the study of the phase structure
of QCD at high temperature and baryon number density by Monte-Carlo
simulations. We find that the sign problem for non-zero density QCD induces a
serious problem in the finite volume scaling analysis of the Lee-Yang zeros for
the investigation of the order of the phase transition. If the sign problem
occurs at large volume, the Lee-Yang zeros will always approach the real axis
of the complex parameter plane in the thermodynamic limit. This implies that a
scaling behavior which would suggest a crossover transition will not be
obtained. To clarify this problem, we discuss the Lee-Yang zero analysis for
SU(3) pure gauge theory as a simple example without the sign problem, and then
consider the case of non-zero density QCD. It is suggested that the
distribution of the Lee-Yang zeros in the complex parameter space obtained by
each simulation could be more important information for the investigation of
the critical endpoint in the plane than the finite volume scaling
behavior.Comment: 16 pages, 3 figures, 2 tables, minor change
Constraints on Dirac-Born-Infeld type dark energy models from varying alpha
We study the variation of the effective fine structure constant alpha for
Dirac-Born-Infeld (DBI) type dark energy models. The DBI action based on string
theory naturally gives rise to a coupling between gauge fields and a scalar
field responsible for accelerated expansion of the universe. This leads to the
change of alpha due to a dynamical evolution of the scalar field, which can be
compatible with the recently observed cosmological data around the redshift
. We place constraints on several different DBI models
including exponential, inverse power-law and rolling massive scalar potentials.
We find that these models can satisfy the varying alpha constraint provided
that mass scales of the potentials are fine-tuned. When we adopt the mass
scales which are motivated by string theory, both exponential and inverse
power-law potentials give unacceptably large change of alpha, thus ruled out
from observations. On the other hand the rolling massive scalar potential is
compatible with the observationally allowed variation of alpha. Therefore the
information of varying alpha provides a powerful way to distinguish between a
number of string-inspired DBI dark energy models.Comment: 11 pages, 6 figure
Anti-D-brane as Dark Matter in Warped String Compactification
It is pointed out that in the warped string compactification, motion of
anti-D-branes near the bottom of a throat behaves like dark matter. Several
scenarios for production of the dark matter are suggested, including one based
on the D/anti-D interaction at the late stage of D/anti-D inflation.Comment: 8 pages, version accepted for publication as a Rapid Communication in
PRD, discussion about mass and production of dark matte
Dynamics of dark energy
In this paper we review in detail a number of approaches that have been
adopted to try and explain the remarkable observation of our accelerating
Universe. In particular we discuss the arguments for and recent progress made
towards understanding the nature of dark energy. We review the observational
evidence for the current accelerated expansion of the universe and present a
number of dark energy models in addition to the conventional cosmological
constant, paying particular attention to scalar field models such as
quintessence, K-essence, tachyon, phantom and dilatonic models. The importance
of cosmological scaling solutions is emphasized when studying the dynamical
system of scalar fields including coupled dark energy. We study the evolution
of cosmological perturbations allowing us to confront them with the observation
of the Cosmic Microwave Background and Large Scale Structure and demonstrate
how it is possible in principle to reconstruct the equation of state of dark
energy by also using Supernovae Ia observational data. We also discuss in
detail the nature of tracking solutions in cosmology, particle physics and
braneworld models of dark energy, the nature of possible future singularities,
the effect of higher order curvature terms to avoid a Big Rip singularity, and
approaches to modifying gravity which leads to a late-time accelerated
expansion without recourse to a new form of dark energy.Comment: 93 pages, 26 figures, Invited Review to be submitted to International
Journal of Modern Physics D; comments are welcome; Additional references
included in response to over 60 comments received. Rewriting of sub-sections
on anthropic principle and gravitational backreaction. New subsections adde
Generation of electromagnetic fields in string cosmology with a massive scalar field on the anti D-brane
We study the generation of electromagnetic fields in a string-inspired
scenario associated with a rolling massive scalar field on the anti-D3
branes of KKLT de Sitter vacua. The 4-dimensional DBI type effective action
naturally gives rise to the coupling between the gauge fields and the inflaton
, which leads to the production of cosmological magnetic fields during
inflation due to the breaking of conformal invariance. We find that the
amplitude of magnetic fields at decoupling epoch can be larger than the
limiting seed value required for the galactic dynamo. We also discuss the
mechanism of reheating in our scenario and show that gauge fields are
sufficiently enhanced for the modes deep inside the Hubble radius with an
energy density greater than that of the inflaton.Comment: 4 pages and 2 eps figures, minor clarifications added and typos
correcte
Inflation and dark energy arising from geometrical tachyons
We study the motion of a BPS D3-brane in the NS5-brane ring background. The
radion field becomes tachyonic in this geometrical set up. We investigate the
potential of this geometrical tachyon in the cosmological scenario for
inflation as well as dark energy. We evaluate the spectra of scalar and tensor
perturbations generated during tachyon inflation and show that this model is
compatible with recent observations of Cosmic Microwave Background (CMB) due to
an extra freedom of the number of NS5-branes. It is not possible to explain the
origin of both inflation and dark energy by using a single tachyon field, since
the energy density at the potential minimum is not negligibly small because of
the amplitude of scalar perturbations set by CMB anisotropies. However
geometrical tachyon can account for dark energy when the number of NS5-branes
is large, provided that inflation is realized by another scalar field.Comment: 11 pages, 8 figure
Prospects of inflation in delicate D-brane cosmology
We study D-brane inflation in a warped conifold background that includes
brane-position dependent corrections for the nonperturbative superpotential.
Instead of stabilizing the volume modulus chi at instantaneous minima of the
potential and studying the inflation dynamics with an effective single field
(radial distance between a brane and an anti-brane) phi, we investigate the
multi-field inflation scenario involving these two fields. The two-field
dynamics with the potential V(phi,chi) in this model is significantly different
from the effective single-field description in terms of the field phi when the
field chi is integrated out. The latter picture underestimates the total number
of e-foldings even by one order of magnitude. We show that a correct
single-field description is provided by a field psi obtained from a rotation in
the two-field space along the background trajectory. This model can give a
large number of e-foldings required to solve flatness and horizon problems at
the expense of fine-tunings of model parameters. We also estimate the spectra
of density perturbations and show that the slow-roll parameter eta_{psi
psi}=M_{pl}^2 V_{,psi psi}/V in terms of the rotated field psi determines the
spectral index of scalar metric perturbations. We find that it is generally
difficult to satisfy, simultaneously, both constraints of the spectral index
and the COBE normalization, while the tensor to scalar ratio is sufficiently
small to match with observations.Comment: 12 pages, 8 figures, version to appear in Physical Review
Relation between fundamental estimation limit and stability in linear quantum systems with imperfect measurement
From the noncommutative nature of quantum mechanics, estimation of canonical
observables and is essentially restricted in its
performance by the Heisenberg uncertainty relation, \mean{\Delta
\hat{q}^2}\mean{\Delta \hat{p}^2}\geq \hbar^2/4. This fundamental lower-bound
may become bigger when taking the structure and quality of a specific
measurement apparatus into account. In this paper, we consider a particle
subjected to a linear dynamics that is continuously monitored with efficiency
. It is then clarified that the above Heisenberg uncertainty
relation is replaced by \mean{\Delta \hat{q}^2}\mean{\Delta \hat{p}^2}\geq
\hbar^2/4\eta if the monitored system is unstable, while there exists a stable
quantum system for which the Heisenberg limit is reached.Comment: 4 page
Solar system and equivalence principle constraints on f(R) gravity by chameleon approach
We study constraints on f(R) dark energy models from solar system experiments
combined with experiments on the violation of equivalence principle. When the
mass of an equivalent scalar field degree of freedom is heavy in a region with
high density, a spherically symmetric body has a thin-shell so that an
effective coupling of the fifth force is suppressed through a chameleon
mechanism. We place experimental bounds on the cosmologically viable models
recently proposed in literature which have an asymptotic form f(R)=R-lambda R_c
[1-(R_c/R)^{2n}] in the regime R >> R_c. From the solar-system constraints on
the post-Newtonian parameter gamma, we derive the bound n>0.5, whereas the
constraints from the violations of weak and strong equivalence principles give
the bound n>0.9. This allows a possibility to find the deviation from the
LambdaCDM cosmological model. For the model f(R)=R-lambda R_c(R/R_c)^p with
0<p<1 the severest constraint is found to be p<10^{-10}, which shows that this
model is hardly distinguishable from the LambdaCDM cosmology.Comment: 5 pages, no figures, version to appear in Physical Review
Generic estimates for magnetic fields generated during inflation including Dirac-Born-Infeld theories
We estimate the strength of large-scale magnetic fields produced during
inflation in the framework of Dirac-Born-Infeld (DBI) theories. This analysis
is sufficiently general in the sense that it covers most of conformal symmetry
breaking theories in which the electromagnetic field is coupled to a scalar
field. In DBI theories there is an additional factor associated with the speed
of sound, which allows a possibility to lead to an extra amplification of the
magnetic field in a ultra-relativistic region. We clarify the conditions under
which seed magnetic fields to feed the galactic dynamo mechanism at a
decoupling epoch as well as present magnetic fields on galactic scales are
sufficiently generated to satisfy observational bounds.Comment: 7 pages, no figure, accepted in Phys. Rev.
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